The present disclosure relates to lapping of spherical objects and specifically to a lapping machine that utilizes patterns of movement of a lapping disc to customize the production of perfectly spherical objects.
Lapping is a process in which surfaces are rubbed together with an abrasive between them, typically performed by a lapping machine. A lapping machine uses an abrasive surface to smooth a workpiece. The most common type of lapping machine currently in use for creating precision spherical shapes utilizes two large discs rotating in parallel, between which the spherical objects are being lapped. One of the lapping discs is typically grooved. During lapping, abrasive slurry is introduced between the upper and lower discs to provide an abrasive action to smooth spherical objects. As a spherical object is lapped, it reduces in size, allowing it to travel down the groove until it exits the lapping machine at a certain size. The dimensions of the lapping disc grooves determine the ultimate size of the spherical workpiece.
The lapping process described above is typically used for mass production of spherical objects. The set up and design of the discs and machines for a typical lapping process is expensive to adjust for shape or size. The high volumes produced in the typical lapping operation allow for a profit that can overcome the high cost of adjusting the equipment for each different batch of spherical objects.
Another type of lapping involves probes of coordinate measuring machines (CMMs). Using coordinate measuring machines requires touch probes which, over time, accumulate materials shredded from the surfaces to be measured. They need to be re-constituted in a precise fashion not feasible with conventional spherical lapping equipment.
Existing lapping machines lack the ability to easily match lapping of spherical components in a ball and socket geometry. Ball and socket geometry is desirable in replacement joint geometries, such as a hip joint, or matched bearing sets for high precision component alignment and high speed rotation. Recent advances in the use of implantable prosthetic joints to replace natural joints in humans have resulted in great advances in treatment of patients. Many designs for such prosthetic implants, particularly for the hip joint, require a spherical head having an extremely smooth surface to reduce friction and increase the life of the joint.
Existing technologies for machining spherical parts on a ball grinder often result in a relatively rough surface. The resulting spherical parts may be buffed to achieve a more polished surface. A problem with currently existing buffing technologies, however, is that surface irregularities, such as grinding and buffing lines, remain on the finished part. Buffing by hand is inferior to machine buffing or lapping, and it is both labor intensive and time consuming, resulting in high costs for the finished spherical product. Additionally, manual buffing and lapping yields lower precision and cannot be considered repeatable processes.